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Article
Metal-Containing Ceramic Composite with In-Situ GrownCarbon Nanotube as a Cathode Catalyst for Anion Exchange
Membrane Fuel Cell and Rechargeable Zinc-Air BatteryPrabu Moni, Maurício Goraiebe Pollachini, Michaela Wilhelm, JulianLorenz, Corinna Harms, M. Mangir Murshed, and Kurosch Rezwan
ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01276 • Publication Date (Web): 17 Jul 2019
Downloaded from pubs.acs.org on July 18, 2019
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Metal-Containing Ceramic Composite with In-Situ Grown Carbon
Nanotube as a Cathode Catalyst for Anion Exchange Membrane Fuel Cell
and Rechargeable Zinc-Air Battery
Prabu Monia, b, Maurício Goraiebe Pollachinia, c, Michaela Wilhelma*, Julian Lorenzd, Corinna Harmsd, M. Mangir Murshede, f, Kurosch Rezwana, f
a University of Bremen, Advanced Ceramics, Am Biologischen Garten 2, IW3, Bremen, Germany
b CSIR-Central Electrochemical Research Institute-Madras unit, CSIR Madras Complex, Taramani, Chennai – 600 113, India
c Federal University of Santa Catarina, Department of Materials Engineering, Florianópolis - SC, 88040-900, Brazil
d DLR Institute of Networked Energy Systems, Department Fuel Cells, 26129 Oldenburg, Germany
e University of Bremen, Institute of Inorganic Chemistry and Crystallography, Leobener Straße 7, 28359 Bremen, Germany
f University of Bremen, MAPEX Center for Materials and Processes, Bibliothekstraße 1, 28359 Bremen, Germany
* Corresponding author. Tel.: +49 421 218 64944; fax: +49 421 218 64932.E-mail address: [email protected]
ABSTRACT
The development of new air-breathing cathode catalyst not only thrust the performance of
fuel cells and metal-air batteries, but also make them cheaper. Herein, we developed a new,
metal containing (Ni, Co, Pt and their alloys) ceramic composite as a cathode electrocatalyst
for anion exchange membrane fuel cell (AEMFC) and zinc-air battery (ZAB) application. The
porous ceramic foams were generated with the help of a sacrificial template method in which
polystyrene beads were infiltrated with a polysiloxane precursor. Addition of metal salts into
the porous ceramic matrix the formation of carbon nanotubes (CNTs) by applying catalyst-
assisted pyrolysis was facilitated. The in-situ grown CNTs in composite ceramic effect the
charge transport and drastically improved the electrical conductivity of up to six orders in
magnitude than the bare ceramics (H.A). The best performing Ni-containing ceramics
(H.A.Ni) show improved oxygen reduction activity in half-cell measurements and for
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AEMFC delivered an open-circuit voltage of 0.65 V with a maximum power density of ~10
mW cm−2. In rechargeable ZAB systems, the H.A.Ni showed excellent battery performance
with a specific capacitance of 490 mAh g−1, maximum power density of 110 mW cm−2, and
excellent discharge/charge cycle stability over 300 cycles. Results indicate that the presence
of CNT and intermetallic silicides such as Ni2Si and Ni31Si12 tune the electrical properties and
enhance the electrocatalytic activity towards oxygen. Thus, the Ni-containing ceramic
material is as an excellent cathode catalyst for AEMFC and rechargeable ZAB applications.
KEYWORDS
Ceramic composite, sacrificial template, in-situ growth of CNT, electrocatalyst, AEMFC,
ZAB
INTRODUCTION
The fuel cells and metal-air batteries are a set of next-generation energy sectors, which
seems to hold notable potential for safe and efficient way to store and convert energy.1–5 In
rechargeable metal-air batteries, the oxygen evolution reaction (OER) and oxygen reduction
reaction (ORR) occur on the cathode side during charge and discharge process, respectively.
During ORR, the oxygen diffuses, is reduced and the produced hydroxide migrates across the
electrolyte, while during OER, the process is reversed and oxygen evolves. Due to the
sluggish kinetics of both ORR/OER, an electrocatalysts is highly essential to facilitate the
ORR/OER reaction pathways. Hence, the electrocatalysts determines to a large extent the
energy conversion or storage efficiency by reducing the overpotential for both ORR/OER and
enhancing the cycle life when the reactant is in a gas phase.6–8 Until now, precious catalysts
such as Pt/C are normally applied as ORR catalyst for fuel cells and Ru/C, Ir/C exhibit
promising potentials for OER based electrolysis, whereas Pt-Ru/C is predominantly used as a
bi-functional catalyst for metal-air batteries. However, the widespread utilization of precious
catalysts is limited by their unaffordable price, the paucity of the noble metals, and poor
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durability.9–11 Current challenges in electrocatalyst research include identifying a new
affordable catalyst, materials processing, understanding unprecedented properties, and use
them.11
Recently, polymer-derived ceramics (PDCs) have been emerging as a prospective aspirant
due to their high thermodynamic and chemical stability. However, the low electrical
conductivity and poor electrocatalytic activity are the main impediment of using them as
electrocatalyst. On the other hand, the possibility of tailoring the electrical and electrocatalytic
properties of PDC, by different precursors, increasing the pyrolysis temperature or adding
conductive fillers make them one of the suitable candidates.12,13 The in-situ growth of Carbon
Nanotubes (CNT) within a porous PDC structure was already reported with the presence of
dehydrogenation-active transition metal nanoparticles by applying catalyst-assisted pyrolysis
(CAP).14–18 For creating porous PDCs, various methods such as foaming, freeze-casting, or
sacrificial templates were widely described.12,19–21 Previous studies in our group demonstrated
that the promising way to get interconnected pore structures is by using the sacrificial
template technique with polystyrene beads (expandable; 98 Vol% air).22 Moreover, one of the
main advantages of using PDCs is that the added metal precursors can be mixed directly with
the polymer precursor, which provides a homogeneous distribution of metal nanoparticles.
With these special features, the PDCs were employed in many applications such as electrode
for energy storage (lithium-ion batteries, sodium-ion batteries, supercapacitors) and solid
adsorbents for gas adsorptions (CO2, SO2 gases).23–31 Recent studies have shown that a metal-
containing ceramic electrocatalysts act as a new class of catalysts for the ORR and OER.32,33
Very recently, cobalt particle promote the CNTs growth in mesoporous carbon has been
reported for Li-O2 air cell applications.34 Interestingly, there have been only a few reports on
transition metal silicide-based catalyst as non-noble metal–based hydrogenation catalyst due
to its unprecedented physical and chemical properties. Within the transition metal silicides,
the dopant Si lower the d band structure and modify the electronic structure around the Fermi
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level of intermetallic metal silicide which further improve the electrical conductivity as well
as chemical and thermal stability.33,35–40 However, to the best of our knowledge, the use of
transition metal silicide-based electrocatalysts have never been investigated for anion
exchange membrane fuel cell (AEMFC) and zinc-air battery (ZAB) application. In this paper,
we describe a preparation method to generate non-precious metal containing hybrid ceramic
foams suitable for energy storage and conversion device. The macro porosity was realized by
using polystyrene beads, which later help the formation of in-situ growth of CNT. Therefore,
the main aim of this work was to ascertain if the novel materials are suitable as cathode
electrocatalysts for AEMFC and ZAB.
EXPERIMENTAL SECTION
Synthesis of ceramic monolith
For the preparation of porous ceramic monolith, the sacrificial template method
followed by catalyst-assisted pyrolysis was used.22 In this preparation process, a silicone resin
poly (methyl phenyl silsesquioxane) (Silres®H44, WackerChemie AG) and 3-
aminopropyltriethoxysilane (APTES, ABCR, Germany) were used as components of the
polymeric precursors. Expanded polystyrene beads (PSB, 0.5 – 1 mm, Fa. Klassen Vitali,
Germany) consisting of 98 vol% air and 2 vol% polystyrene was used as sacrificial templates.
The metal salt such as nickel chloride (NiCl2), cobalt chloride (CoCl2), and chloroplatinic acid
(H2PtCl6·6H2O) was used as metal source. The calculated amount of H44, APTES and metal
salt were separately mixed with ethanol and stirred for 3 hours to ensure a complete
dissolution. The molar ratios of H44 : APTES and APTES : metal salt were optimized in a
previous study and values of 92 : 8 and 8 : 1 respectively, were used.22 After the complete
dissolution of each set, these solutions were mixed together and stirred. The sacrificial
templates (polystyrene beads) were filled in a medical syringe based on a volume
compression and dimension of the sample. Then the metal salt-containing polymer solution
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was infiltrated into the medical syringe and compress by 6.25% of the volume.22 This
compression was chosen since it allows for achieving a porous structure. After three days of
cross-linking at room temperature, the samples were taken out from the syringe and thermally
cross-linked at 100 °C for 24 h followed by final pyrolysis at 1000 ºC for 4 h in an
atmosphere of nitrogen. The samples were named as H.A, H.A.Ni, H.A.Co, H.A.Pt,
H.A.Ni.Co, H.A.Ni.Pt, and H.A.Co.Pt respectively, whereas the letter H stands for poly
(methyl phenyl silsesquioxane), A for APTES, Ni for nickel, Co for cobalt, and Pt for
platinum precursors. Detailed synthesis procedure and schematic view of a monolithic
structure were presented in Fig. S1, S2 (Supporting Information) and Table S1 (SI).
Material characterization
X-ray diffraction pattern of the ceramic composite was obtained by using a Seifert
XRD powder diffractometer (Cu-Kα radiation, ID 3303, General Electric, USA). Raman
spectra were recorded on a LabRAM ARAMIS (Horiba Jobin Yvon) with a laser working at
633 nm with less than 20 mW. The decomposition behaviour of ceramic precursors was
measured with a thermogravimetric analyzer (STA 503 BÄHR). The surface morphology,
CNT formation and metal particle distributions were investigated with the help of FESEM
(ZEISS Supra 40, Oberkochen, Germany) and FETEM (FEI Titan 80, 300 kV) respectively.
The specific surface area and pore size distribution of ceramic composite was calculated by
using nitrogen adsorption/desorption isotherms with a Belsorp-Max (Bel Japan Inc.) and
mercury intrusion porosimetry (Pascal 140/440 POROTEC GmbH) respectively. For
conductivity measurements, AC impedance studies were carried out with ceramic pellets (≈ 1
mm of thickness and 10 mm diameter) using stainless steel as blocking electrodes and a
computer-controlled electrochemical analyzer (IM6ex Zahner® Elektrik) in the frequency
range of 1 MHz to 10 mHz at room temperature.
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Electrocatalytic measurements
The bi-functional activities (ORR/OER) of ceramic catalyst were tested with a help of
computer-controlled electrochemical workstation (Biologic, VSP/VMP 3B-20, France)
interfaced with a rotating ring-disk electrode apparatus (RRDE-3A) using the linear sweep
voltammetry and cyclic voltammetry (LSV & CV) method. The specific electrocatalytic
characterization details were reported in the previous literature.33 A platinum wire, saturated
calomel electrode and catalyst coated glassy carbon were acted as the counter, reference and
working electrodes, respectively. The catalyst inks were prepared by suspending 3.55 mg
ceramic catalyst into a mixture of 150 µL of DI water, 250 µL ethanol and 10 µL 5wt%
Nafion ionomer solution. A homogenous coating of catalyst ink was achieved by pipetting 4
µL ink on a glassy carbon disc electrode to obtain a loading of 500 µg cm−2 and
electrocatalytic activities were tested in 0.1 M KOH electrolyte solution under oxygen
saturated condition (before N2 saturated electrolyte was used for background correction).
State of the art ORR catalyst Pt/C (40%) with the loading of 60 µg/cm2 and OER catalyst
RuO2 with the loading of 500 µg/cm2 were also tested for reference.
AEMFC assembly and testing
In the AEMFC, the commercial gas diffusion layer (GDL, SGL DC-35) was used as a
backing layer for both anode and cathode. The cathode catalyst ink was prepared by
dispersing 8 mg of the ceramic composite in 0.8 mg of 10 wt% Fumion FAA-3 ionomer and
suitable amount of solvent under ultrasonication. Then the catalyst ink was coated on the
GDL using brush coating technique with the catalyst loading of 2 mg cm−2. The anode was
prepared by coating of a commercial Pt/C (40%) on a GDL with a loading of 0.5 mg cm−2.
Both electrodes were immersed in 1 M KOH solution for 12 hours to replace the Br- ions by
OH- ions. The commercial Fumapem membrane (FumaTech) with a thickness of 50 μm was
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used as membrane after immersed in 2 M KOH solution for 12 hours to replace the existing
Cl- ions by OH- ions at room temperature.41,42 Fuel cell was constructed by in-situ assembly
of a membrane sandwiched between the cathode (4 cm2 active area) and anode in a
commercially available fuel cell hardware (Fuel cell Tech. Inc., USA) with parallel serpentine
flow field machined on graphite plates. During the measurements, the reactants H2 and O2
with 100% relative humidity (RH) were fed into the anode and cathode side at a flow rate of
100 and 150 mL min-1, respectively. The galvanostatic polarization studies were conducted
using LCN 100-36 electronic loads from Bitrode Corporation, USA. Before the fuel cell perfor-mance evaluation, the MEA was stabilized at 100% RH condition for4 h.Before the fuel cell perfor-mance evaluation, the MEA was stabilized at 100% RH condition for4 h.Before the fuel cell perfor-mance evaluation, the MEA was stabilized at 100% RH condition for4 h.Before the fuel cell perfor-mance evaluation, the MEA was stabilized at 100% RH condition for4 hZAB assembly and testing
The ceramic catalyst containing ZAB was assembled in a homemade setup (Fig. S3).
In a typical procedure, catalyst ink was prepared by dispersing 6 mg of ceramic catalyst in 12
mg of 5wt% Nafion ionomer and suitable amount of ethanol under constant ultrasonication.
Then the catalyst ink was coated on the GDL (SGL DC-35) by using brush coating technique
with a catalyst loading of 1 mg cm−2, which is explored as cathode. A polished zinc plate,
polypropylene (PP) membrane, and 6 M KOH alkaline solution were used as the anode,
separator and electrolyte, respectively. Before constructing the cell, the polypropylene
separator was immersed in electrolyte solution for 12 hours to enrich OH- ions. In-situ
assembly of electrolyte-enriched separator sandwiched between the zinc-plate and catalyst-
coated GDL, can assemble ZAB can be assembled. The galvanostatic response of ZAB was
performed by consuming air from the environment by using an electrochemical workstation
(Biologic, VSP/VMP 3B-20, France). The specific capacity of primary ZAB was calculated
based on the mass of zinc consumed during discharge. Polarization and power density curves
of the ZAB were analyzed by using LCN100-36 electronic loads from Bitrode Corporation-
USA, by feeding air from the environment at room temperature and ambient pressure.
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RESULTS AND DISCUSSION
Structure and morphology
To produce porous ceramic monoliths, the sacrificial template method is the promising
way to get foam structures with interconnected pores, which are essential for catalytic
supports used in fuel cells and metal-air battery, where the reactant is in the gas phase. The
addition of a metal salt into a ceramic matrix result in a formation of electrocatalytically
active species with the help of catalyst-assisted pyrolysis. The XRD pattern of bare (H.A) and
metal-containing ceramics (H.A.Ni and H.A.Co) pyrolyzed at 1000 °C are shown in Fig. 1a.
The bare H.A shows the typical H44 non-crystalline ceramic structure with two broad halos at
23 and 43o. The metal-containing ceramics show a strong diffraction peak at 26.2o related to
graphitic carbon phase and not showing any H44 peaks.15 In addition to these observations,
the Ni-containing ceramics show the reflexes of intermetallic nickel silicide such as Ni2Si and
Ni31Si12.33 For Ni-containing ceramics even at 1000 °C, the Ni strongly favors the reduction
of Si-O bonds and the forthright formation of nickel silicides. When the pyrolysis temperature
increases, the diffusion rate of the Si atom into metallic Ni gradually increases, the dopant Si
modify the metal coordination in nickel silicides, leading to a downshift of the d band center
and a strong alteration of the electronic structure nearby the Fermi level.37,39,40 At 1000 °C the
small amount of Si atoms distributed into the nickel rich lattice results in a mixture of
Ni2Si/Ni31Si12 with 28-33 at% Si.40,43 However, in case of cobalt-containing ceramics, only
the diffraction peaks of metallic cobalt particles are found. The formation of intermetallic
cobalt silicide is rather hindered at 1000 °C, since a further increase in temperature would be
required for reduction of Si–O bonds and the formation of intermetallic cobalt silicides.40
Thus, the interaction between the Co and Si is somewhat limited, what causes a lower weight
loss (Fig. S4). The XRD patterns of H.A.Pt, H.A.Ni.Pt and H.A.Co.Pt was almost similar to
that of bare H44, indicating the non-crystalline nature (Fig. S5a). Whereas Pt-containing
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ceramics shows few peaks of intermetallic platinum silicide (Pt2Si). Hence, the addition of Pt
and its alloys did not influence the formation of graphitic carbon. Apart from crystallinity, the
ordered/disordered nature of carbon present in the ceramic matrix was investigated with the
help of Raman spectroscopy (Fig. 1b, Fig. S5b).44 All ceramics show the first order sharp
peak at ~ 1332 cm-1 assigned as D peak caused by the disordered structure of the graphite
structure in the ceramic matrix. The presence of E2g mode peak at ~ 1595 cm-1 assigned as G
peak is due to the in-plane bond stretching of sp2 carbon atoms within the ceramic composite.
An increase in IG/ID intensity ratio observed for PDC with the addition of Ni, Co, Ni-Pt and
Co-Pt confirm the presence of curved and closed graphitic structures in nanotubes forms.45 In
addition, the strong appearance of a 2D peak at ~ 2673 cm-1 for Ni, Co, Ni-Pt, and Co-Pt
containing ceramics shows the existence of graphene layer confirming the higher degree of
CNT formation.46–48
In this sacrificial template method, the polystyrene beads were completely dispersed in a
continuous polymeric solution. During the thermal process, the complete removal polystyrene
beads create a foam type monolithic structure (Fig. S4).22 Fig. 2 shows the FESEM images of
bare (H.A) and metal-containing ceramics pyrolyzed at 1000 °C. The ceramic monolith shows
a foam type structure with interconnected spherical cells having diameters in the range of 0.5–
1.0 mm. The regular arrangement of polystyrene beads in the mold and optimal compression
(6.25% vol.) applied during the process helps the beads to easily deform and produces
spherical, interconnected macropores in each side of the monolith (Fig. S6). It is already
reported that in catalyst-assisted pyrolysis the in-situ growth of CNT is possible in porous
PDC, with the help of a metal catalyst.15,16,43,49 The metal (Ni and Co)-containing ceramics
pyrolyzed at 800 °C show the beginning of CNT formation in pores and formation is more
pronounced when the pyrolysis temperature increased to 1000 °C (Fig. 2(c-e), Fig. S7).
However, in the case of bare H.A, the formation of CNTs at both temperatures cannot be
observed (Fig. 2(a, b), Fig. S7). The formation of CNTs in Ni/-Co containing PDC is
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facilitated by various factors:15,16 (i) the metal-containing ceramics show a decrease in
activation energy for soot oxidation as a result of metal (intermediate oxide) transfers oxygen
to soot particles; (ii) the electronic interaction between π state of carbon atoms (sp2) with the
metal or metal oxide leads to a less delocalized electronic state of carbon atoms due to higher
electronegativity of carbon (2.55) compared with transition metal (Ni (1.91); Co (1.88)); and
(iii) the addition of Ni/-Co based catalyst facilitate the dehydrogenation reaction that involves
the removal of hydrogen from a PDC. These possible mechanisms for the formation of CNTs
in metal-containing PDC have been presumed from similar material described in various
literature.15,16,50,51 Moreover, the reaction started at gas phase promotes the in-situ growth of
CNTs, whereas in the solid matrix results in turbostratic carbon.43 The grown CNTs show at
least 13–15 layers (MWCNTs) with inner and outer diameters of 10−15 and 30−60 nm,
respectively, with a length of a few microns (Fig. 2-4). The FETEM image shows that the
intermetallic silicide’s or metal nanoparticles were nicely embedded in the ceramic matrix
with an average particle size of about ∼20 nm (Fig. 3(a, c)).43 The EDX spectrum confirms
the high atomic percentage of carbon about 69 % for H.A.Ni when compared with bare H.A
(31.6 %) (Fig. S8, S9). In addition, the ceramic samples with Ni-Co alloys (H.A.Ni.Co) still
show the formation of CNTs (Fig. 4(a, b)). However, the Pt-containing ceramics (H.A.Pt) did
not show any formation of CNTs, instead agglomerated carbon particles are observed along
with intermetallic platinum silicide spheres (Fig. 2f). The possible reasons for hampering
CNT formation in Pt-based precursors at 1000 °C is due to (i) the higher activation energy for
soot oxidation as a result of limited interaction of Pt with subsurface “oxide” formation at this
temperature and there is no oxygen transfers to soot particles; and (ii) the electronegativity of
Pt (2.28) is very similar to carbon atoms (2.55), which affects the delocalized electronic state
of carbon atoms. These tentative hypothesis have been deduced from previous studies of
Ni/Co-containing PDC materials.15,16,50,51 It is believed that further studies are necessary to
explain the role of Pt catalyst in the oxidation process. Interestingly, the Ni-Pt (H.A.Ni.Pt) and
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Co-Pt (H.A.Co.Pt) containing ceramics possess a jellyfish-like morphology (Fig. 4(c-f)). The
EDX mapping analysis at various spots of jellyfish like morphology (H.A.Ni.Pt) shows the
presence of Si, Ni, Pt and oxygen in the core and carbon at the shell, ensuring that the
formation of spherical Pt-Ni-Si alloys at the solid matrix and CNTs at gas phase (Fig. S10).
The Ni-Pt and Co-Pt ceramic samples having various metals such as nickel, cobalt and
platinum having different electronegativity, dehydrogenation properties and surface “oxide”
results in a jellyfish-like morphology. Overall, the surface morphology analysis confirms that
with the existence of pores in PDC the addition of Ni and Co was beneficial for an in-situ
growth of CNTs.
Moreover, the thermal stability of the metal-containing ceramics is slightly better than that
of bare H.A as a result of in-situ grown CNT, which improve the overall thermal stability of
the ceramic composite (Fig. S4).52 The in-situ grown CNTs influence the BET specific
surface and open porosity were shown in Fig. S12. The bare ceramic (H.A) sample show a
Type II isotherm with negligible N2 adsorption and SSA value as close as 2 m2g-1. The metal
(Ni/Co)-containing ceramic composite show a Type IV hysteresis with a loop starting at lower
relative pressures (p/po~0.45). The Ni-containing ceramic samples show the highest open
porosity 22% (calculated from mercury intrusion porosimetry) and SSA of 9 m2g-1 (calculated
from N2 adsorption), respectively. The existence of mesopores result from spaces between the
in-situ grown CNT and the cell walls.14
Electrical conductivity
The grown CNT influences the electrical properties of ceramic composite was
analyzed by applying an AC (alternate current) impedance technique. From the conductance
spectrum (Fig. 5a), it is evident that at low frequencies, conductivity shows a frequency-
independent trend, what describes the DC (direct current) conductivity of the material. When
the frequency increases the conductivity exhibits AC conductivity behavior.53 For the bare
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ceramics (H.A) at lower frequency region (100 Hz), the DC conductive behavior changes to
AC conductivity and then AC dispersion pattern observed describing the semiconducting
behavior of H.A. However, for Pt or Co-containing ceramics the frequency where the DC
conductivity changing to AC conductivity is shifting at very high frequency (1 MHz) and for
Ni-containing ceramics the shifting region almost disappears due to the highly conductive
nature of the ceramics (Fig. 5a). Fig. 5b shows the DC conductivity (σDC) of ceramic
composite calculated at room temperature. Pure H.A pyrolyzed at 1000 °C shows the σDC
value of 5x10−10 S cm−1. For the metal-containing ceramics, the σDC values increase by three
to six orders of magnitude. The highest conductivity value of 4.9x10−4 S cm−1 was observed
for Ni-containing ceramics and it was found to be almost one order higher than Co-containing
ceramics (4.4x10−5 S cm−1) due to a prominent growth of CNTs.52 Further, during the
formation of intermetallic nickel silicide, the dopant Si modifies the electronic structure
around the Fermi level of intermetallic silicide that further improve the electrical conductivity
of the ceramic composite. For Pt-containing ceramics, the value drops to 2.5x10−6 S cm−1,
since the addition of Pt did not show any CNT formation at 1000 °C. In addition, the Pt-based
bimetallic ceramics still show a drop in conductivity due to the limited formation of the CNT.
The earlier studies showed that the electrical conductivity of Graphene or CNT incorporated
H.A are still unsatisfying specifically in terms of their low electrical conductivity (~10-5 S cm-
1 even after 10 wt% nanofillers incorporation) because of their agglomeration issues, which
eventually hinders their conductivity.52 However, the Ni-containing ceramics shows the
highest electrical conductivity because of in-situ grown CNT, which are uniformly embedded
into the ceramic matrix.
Electrocatalytic activity
The bi-functional activity of metal-containing ceramic catalyst towards both ORR and
OER was assessed by using a CV and LSV technique in alkaline media (Fig. 6 and Fig. S12-
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S13). The effect of various metal-containing ceramic catalysts on electrochemical activity is
investigated (Fig. S12-S13) which clearly indicates that the Ni and Co-containing ceramics
shows better electrochemical activity due to its prominent growth of conductive CNTs and
formation of catalytic active intermetallic silicides. The CV graph of best performing H.A.Ni
exhibits a well-defined oxygen reduction peak in the O2-saturated electrolyte, whereas no
such peak was observed in N2-saturated electrolyte, suggesting their noticeable ORR activity
(Fig. 6a). To investigate the detailed ORR reaction kinetics, LSV analysis was measured at
various rotation speeds from 400 to 2500 rpm. The ORR polarization curves of Ni and Co-
containing ceramic catalyst were shown in Fig. 6(b, c). Among all, the H.A.Ni exhibits the
highest activity with an onset potential of 0.8 V, a half-wave potential of 0.58 V, and a
limiting current density of 3.6 mA cm−2 respectively at 1600 rpm in the ORR region,
indicating improved ORR kinetics compared to H.A.Co. The average number of electrons (n)
involved in ORR calculated from K-L plots for H.A.Ni is 3.2, which is higher than that of
H.A.Co, confirming that ORR proceeds via the mixed 2- and 4- electron pathway. As for OER
activity, H.A.Ni exhibits a sharp increase in current density and generates a current density of
10 mA cm−2 at a potential of 1.78 V as shown in Fig. 6d. This result strongly suggests the
superior OER activity observed for H.A.Ni compared to Co-containing ceramics.
Additionally, the comprehensive bi-functional oxygen activity of the ceramic catalyst was
calculated by the potential difference between the region where OER current density reaches
10 mA cm−2 and ORR half-wave potential (ΔE = Ej=10 − E1/2).54–56 As shown in Fig. 6d, the
H.A.Ni show the lowest ΔE value of 1.20 V, indicating a much improved bi-functional
activity toward both ORR and OER. All results highlight that the importance of Ni-containing
ceramics with the synergistic effect of intermetallic nickel silicides (Ni2Si and Ni31Si12) and
in-situ grown CNT, accounting for the improvement in bi-functional activity. Considering the
structures of Ni2Si and Ni31Si12, as reflected in XRD, it is assumed that the intermetallic
nickel silicides may play a significant role in promoting the ORR/-OER activity of the Ni-
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incorporated ceramic catalyst. Moreover, it is believed from the previous studies, the presence
of intermetallic silicide could improve the catalytic activity, due to its optimum adsorption
energy with reaction intermediates of oxygen during both ORR/-OER.33 Further in-situ grown
CNT embedded in ceramic matrix boosts the electron/ion transport. Even though the Ni-
containing ceramics still show a moderate ORR activity compared with Pt/C, and OER
activity compared with RuO2, but as a bi-functional catalyst it outperforms the state-of the art
Pt/C and RuO2.
AEMFC analysis
Considering the noticeable ORR activity in alkaline media, the best-performing metal-
containing ceramic catalyst was further predestinated as cathode catalyst in AEMFC. The
AEMFC was constructed with the help of H.A.Ni and H.A.Co, and polarization studies were
conducted by using H2 and O2 as reactants at ambient temperature and pressure (Fig. 7). As
for AEMFCs activity, the H.A.Ni shows an open-circuit voltage (OCV) of 0.65 V and a
maximum power density of ~10 mW cm−2 that is superior to the Co-containing ceramics. As
anticipated, the Ni-containing ceramic catalyst show improved fuel cell performance due to
the synergistic effect between intermetallic nickel silicide and grown nanostructured CNT.
The AEMFC performance of various non-precious catalysts were summarized in Table S2.
Although the AEMFC performance of the Ni-containing ceramic catalyst is less than that of
recently reported values, it is still comparable under similar condition using commercial
Fumapem membrane (FumaTech).41,42,57,58 However, the AEMFC performance of these
materials will be tested with suitable membranes for realistic performance.
ZAB analysis
Finally, the feasibility of the metal-containing ceramic air cathode is demonstrated in a
homemade ZAB setup and its performance was tested by breathing open-air as a fuel. Fig. 8a
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displays the polarization and power density curves with best performing H.A.Ni and H.A.Co
as cathode catalyst. The polarization profiles reveal that H.A.Ni shows an OCV of 1.48 V and
a maximum discharge power density of 110 mW cm−2 when the current density reaches 150
mA cm−2. In contrast, Co-containing ceramics show an OCV of 1.41 V and a maximum
discharge power density of 71 mW cm−2 at a current density of 125 mA cm−2. A maximum
power density of 110 mW cm−2 is observed for H.A.Ni, which is much higher than the
benchmark Pt/C + Ir/C catalyst (power density of 90.6 mW cm−2) reported in the
literature.59,60 The primary ZAB was tested by galvanostatically discharging at a constant
current density of 2 mA cm−2 until the voltage reaches 0.5 V. During the deep discharge,
H.A.Ni displays an excellent performance without any voltage fading over 30 h, and
delivering a specific capacity of 490 mAh g−1 (Fig. 8(b, c)). These results clearly confirm the
excellent ORR activity of H.A.Ni in a ZAB is possibly due to the active sites of intermetallic
nickel silicides in the alkaline media, which promotes the excellent ORR reaction kinetics.33
For the realization of electrically rechargeable metal-air batteries, the H.A.Ni based ZAB was
galvanostatically cycled at a constant current density of 2 mA cm−2 with a 10-min cycle
period (Fig. 8d & Fig. S14). The H.A.Ni affords an initial discharge and charge voltage of
1.26 V and 2.04 V respectively, with a voltage gap of 0.78 V, which slightly changed to 1.23
V and 2.02 V with a voltage gap of 0.79 V after 300 cycles. The round-trip voltaic efficiency
of H.A.Ni was calculated to be 62 % of the first cycle and maintained at 61 % over 300
cycles. Hence, the voltage gap of Ni-containing ZAB increases only 10 mV after 300 cycles,
which confirms the excellent long-term cycling stability. Further, to investigate the C-rate
aspects, the ZAB was discharged at various current rates (2 - 40 mA cm−2). The discharge
potential plateau of H.A.Ni shifts downstream as the discharge current rate increases because
of low ORR reaction kinetics. However, the discharge plateau remains stable for each current
density applied and maintains the voltage plateau of 1.13, 0.98, 0.87 and 0.66 V for the
current density of 5, 10, 20 and 40 mA cm−2 respectively. Even after high discharge rate,
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during current step-up, the voltage plateau almost remains the same compared with the initial
current step-down process (Fig. 8e). Hence at a higher current rate, the Ni-containing ceramic
catalyst maintain the voltage plateau without any potential drop or damage indicating the
better electrocatalytic activity and improved mass transfer efficiency.61,62 The ZAB
performance of the Ni-containing ceramic catalyst is comparable to that of recently reported
non-precious catalysts for ZAB and values were summarized in Table S3. Overall, the Ni-
containing ceramics are not only showing high energy density and power density, but also
show better rechargeability and battery’s rate performance is possibly due to following
aspects: i) the structure conversion from non-crystalline H44 structure to graphitic carbon
phase within the ceramic structure, which enhance the chemical and thermal stability of the
ceramic composite; ii) the in-situ grown CNT embedded into the ceramic matrix can provide
efficient electron/ion transportation pathways and improves the overall electrical conductivity;
iii) the presence of intermetallic nickel silicides (Ni2Si and Ni31Si12) with favorable electronic
structure can effectively enhance the electrocatalytic activity of ceramic composites towards
oxygen. With further improvements in the synthesis route, we believe this metal-containing
PDC could be used as a next generation non-precious electrocatalyst for AEMFC and ZAB
applications.
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CONCLUSION
In summary, new cost-effective metal-containing PDCs as an air-breathing cathode
catalyst for AEMFC and ZAB applications are reported. The metal-containing ceramic
monoliths were generated with the help of polystyrene beads as sacrificial templates. Due to
its porous foam architecture and with the help of a catalyst-assisted pyrolysis, the addition of
metal salt facilitates the in-situ growth of CNTs, which further enhances the electrical
conductivity by six orders of magnitude. The Ni-containing ceramics outperform other metal-
based ceramic catalyst and show the better bi-functional (ORR and OER) catalytic activities.
Notably, the presence of intermetallic nickel silicides (Ni2Si and Ni31Si12) effectively
enhances the electrocatalytic activity towards oxygen. As a result, the cathode catalyst in
AEMFC delivers a peak power density of ~10 mW cm−2 under ambient condition. Moreover,
as a cathode catalyst in primary ZAB it delivers a high specific capacitance of 490 mAh g−1,
and the maximum power density of 110 mW cm−2, respectively. For rechargeable ZAB, the
H.A.Ni exhibits small discharge/charge voltage gap, better rechargeability and stable rate
performance by consuming open air from the atmosphere. Most importantly, this work
highlights the importance of metal-containing PDCs, which could shed light on the
development of non-precious cathode catalyst for next-generation AEMFCs and ZABs.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at
http://dx.doi.org/ Sample variation, Process scheme, Schematic view, TGA, XRD, Raman
spectrum, FESEM, EDS, N2 adsorption-desorption isotherms, mercury intrusion porosimetry,
electrocatalytic ORR-OER results, schematic view of home-made ZAB set-up, rechargeable
ZAB performance, AEMFC and ZAB performance of reported catalyst comparison.
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AUTHOR INFORMATION
Corresponding Author
Michaela Wilhelm *E-mail: [email protected]
ORCID
Prabu Moni 0000-0003-2389-5292
Maurício Goraiebe Pollachini 0000-0003-1772-5086
Julian Behnken: 0000-0002-9936-7667
Michaela Wilhelm: 0000-0001-8651-1546
M. Mangir Murshed: 0000-0002-9063-372X
Kurosch Rezwan: 0000-0002-7318-1119
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This research work was supported by Deutsche Forschungsgemeinschaft (DFG) within the
Research Training Group GRK 1860 “Micro-, meso- and macroporous nonmetallic Materials:
Fundamentals and Applications” (MIMENIMA). One of the authors, Dr. Prabu Moni
grateful to the Department of Science and Technology (DST), New Delhi, India for
awarding INSPIRE Faculty Award (DST/INSPIRE/04/2016/000530).
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Fig. 1 (a) XRD; and (b) Raman patterns of ceramic monoliths. H.A; H.A.Ni; and. H.A.Co
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Fig. 2 FESEM images of ceramic monoliths. (a-b) H.A; (c-d) H.A.Ni; (e) H.A.Co; and (f) H.A.Pt
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Fig. 3 FETEM images showing the nanoparticle distribution and CNT formation within the ceramics. (a, b) H.A.Ni, and (c, d) H.A.Co
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Fig. 4 FESEM images of ceramic monoliths (a, b) H.A.Ni.Co; (c, d) H.A.Ni.Pt; and (e, f) H.A.Co.Pt
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Fig. 5 (a) Conductance spectra of ceramic composite recorded at room temperature; and (b) Corresponding room temperature DC conductivity
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Fig. 6 Electrocatalytic activities of metal-containing ceramics in 0.1 M KOH electrolyte solution at a scan rate of 5 mV s−1. (a) CV for H.A.Ni. Oxygen reduction polarization for (b) H.A.Ni; (c) H.A.Co at various rotation; insets show K-L plots, (d) Oxygen electrode activity of metal-containing ceramics and compared with Pt/C and RuO2 at 1600 rpm
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Fig. 7 Polarization and power density curves of AEMFC comprising H.A.Ni and H.A.Co as cathode catalysts in H2/O2 feeds at 30 °C and ambient pressure under 100 % RH
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Fig. 8 ZAB performance of metal-containing ceramics by consuming open air. (a) Polarization and power density curves; (b, c) Primary battery performance; (d) Rechargeable battery performance; (e) Discharge profiles from low current to high current densities
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